This doesn't have anything to do with the move to SQL Server 2008. You need to convert any non-unicode strings to unicode before they reach the destination in your data flow (if it doesn't work even though you've used a data conversion step, check that you're definitely mapping the converted strings into the destination rather than the original ones as it won't replace them).

A good starting place would be to use the import/export wizard in SQL Server Management Studio to export some data, chosing Excel as your destination and then choose to save to an SSIS package, so you can see how the table creation and conversion works.